Structural studies showed that the Streptococcus gordonii 38 receptor polysaccharide was composed of a repeating heptasaccharide linked end to end by phosphodiester bonds. The heptasaccharide repeating units of strain 38 and S. mitis J22 differed only at their reducing ends with GalNAcbeta13Gal in the former and Gal- beta13GalNAc in the latter while a rhamnose branch near the nonreducing end of the strain 38 heptasaccharide distinguished this structure from the linear hexasaccharide of S. oralis 34. Certain strains of S. sanguis and S. gordonii that express GalNAc-sensitive lectins coaggregated with strains 38 and 34 which have GalNAc- beta(13)Gal-containing receptor polysacchides but not with S. mitis J22 which has a Gal-beta13GalNAc containing receptor polysaccharide. Presumably, the complimentarity of these lectins involves the side of GalNAc-beta13Gal that includes the acetamido group of GalNAc-beta. In contrast, actinomyces interact with both types of receptor polysaccharides which suggests lectin complimentarity for a side of GalNAc-beta13Gal that is shared with Gal-beta13GalNAc. The antigenic properties of these polysaccharides appear to depend on structural features distinct from those detected by lectin binding. For example, the presence of the common rhamnose branch accounts for the strong cross reactivity between the strain 38 and J22 polysaccharides. A structural understanding of these cross reactions provides a rational basis for the preparation of specific immunological reagents. Studies were also initiated to characterize the adhesive properties of viridans streptococci that colonize the human oral cavity shortly following birth and also following the eruption of teeth. Whereas isolates obtained during the first two months were relatively nonadherent, isolates obtained after tooth eruption exhibited an array of adhesive properties similar to those noted in studies of adult isolates. The results clearly implicate specific bacterial adhesive properties as important determinants of in vivo colonization.